The invention relates to a system for charging an energy store and a method for operating the charging system according to the invention.
It appears that electronic systems, which combine new energy storage technologies with electric drive technology, will increasingly be used in the future in stationary applications, such as, e.g., wind turbines, as well as in motor vehicles, such as hybrid and electric vehicles. In conventional applications, an electric machine, which, e.g., is embodied as an induction machine, is controlled via an electrical energy converter in the form of an inverter. A so-called DC link, via which an energy store, typically a battery, is connected to the DC side of the inverter, is characteristic of such systems. In order to be able to meet the requirements for performance and energy necessary for any given application, a plurality of battery cells is connected in series. Because the current provided from such an energy store has to flow through all of the battery cells and a battery cell can conduct only a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current.
Besides having a high total voltage, the series connection of a plurality of battery cells poses the problem that the entire energy store fails if a single battery cell fails. As a result, battery current can no longer flow. Such a failure of the energy store can lead to a failure of the total system. In the case of a motor vehicle, a failure of the drive battery can lead to a breakdown of the vehicle. In other applications, such as, e.g., the rotor blade adjustment of wind turbines, situations which endanger safety can even arise when outside conditions are unfavorable, such as, e.g., when strong winds prevail. It is therefore always the goal to achieve a high degree of reliability of the energy store, wherein “reliability” refers to the capability of a system to operate in an error-free manner for a predetermined amount of time.
In the earlier German patent applications DE 10 2010 027857 and DE 10 2010 027861, batteries comprising a plurality of battery module lines are described which can be directly connected to an electric machine. The battery module lines have a plurality of battery modules connected in series. Each battery module comprises at least one battery cell and a corresponding controllable coupling unit, which allows said module to disconnect the respective battery module line or to bridge the at least one respective corresponding battery cell or connect said at least one respective corresponding battery cell into the respective battery module line as a function of control signals. By suitably actuating the coupling units, e.g. with the aid of pulse width modulation, suitable phase signals for controlling the electric machine can also be provided so that a separate pulse width modulated inverter can be eliminated. The pulse width modulated inverter required for controlling the electric machine is thereby for all intents and purposes integrated into the battery. For the purpose of disclosure, these two earlier applications are completely incorporated into the present application.